A fiber laser device is known as one of laser devices used in the fields of processing machines, medical devices, measurement devices, and the like. In such a fiber laser device, light emitted from an amplification optical fiber is sometimes propagated to a desired place through a delivery fiber and then emitted. The light emitted from the delivery fiber is sometimes condensed at a lens, or the wavelength of the light is sometimes converted to the short wavelength side using a wavelength conversion element. In these cases, from the viewpoint of improving light condensing properties or from the viewpoint of improving wavelength conversion efficiency, it is preferable to emit light of excellent beam quality from the delivery fiber that the power of light in a fundamental mode (LP01 mode) is great and the power of light in a higher mode is decreased.
For this delivery fiber, a single mode fiber that propagates only light in a fundamental mode, for example, can be named. However, since the single mode fiber generally has a small effective cross sectional area (Aeff) of light, even though light of great power is to be propagated, a phenomenon such as a nonlinear optical effect caused by an increase in the power density of light occurs, and it is difficult to propagate light of great power. Therefore, an attempt is made that the effective cross sectional area of light is expanded while suppressing the propagation of light in a higher mode and propagating light in a fundamental mode. For an optical fiber used for this attempt, a photonic bandgap fiber can be named in addition to an optical fiber in which a cladding surrounds the outer circumferential surface of a core through which light is propagated.
Patent Literature 1 mentioned below describes a photonic bandgap fiber like this. In this photonic bandgap fiber, a large number of high refractive index portions are disposed in a triangular lattice shape so as to form an outer shape in a regular hexagon. However, these high refractive index portions are not disposed at the position in the center of the regular hexagon at which a high refractive index portion is to be disposed and the positions at which six high refractive index portions are to be disposed to surround the center high refractive index portion, and a core region is formed of a region including the positions at which these high refractive index portion are not disposed. In other words, the photonic bandgap fiber is a so-called seven-cell core type photonic bandgap fiber in which the core region is formed as seven high refractive index portions are not disposed. The photonic bandgap fiber is configured in which the propagation of light in a higher mode is regulated by the difference between the bending losses of light in a fundamental mode and light in a higher mode caused by bending in the state in which the photonic bandgap fiber is bent at a predetermined bending radius, and only light in the fundamental mode is substantially propagated.
[Patent Literature 1] JP-A-2012-73389